It is well known that photographic sensitivity is increased by increasing the grain size of silver halide grains. In order to increase the grain size of silver halide crystals, a method using a silver halide solvent which accelerates the growth of silver halide crystals during precipitation or physical ripening of the silver halide is generally employed.
The silver halide solvents which can be used include nitrogen-containing silver halide solvents, the nitrogen atom of which accelerates the growth of grains by coordination to silver ions, such as ammonia; sulfur-containing silver halide solvents, the sulfur atom of which accelerates the growth of grains by coordination to silver ions, such as thioether compounds, thione compounds (hereinafter described in detail), thiocyanates; and the like.
Of these silver halide solvents, the nitrogen-containing compounds such as ammonia lose the coordination to silver ions upon neutralization with acids, thereby inactivating their effect to increase the growth of grains of silver halide crystals. In other words, ammonia is a convenient silver halide solvent since it functions to accelerate the growth of grains only in required stages and can be inactivated by neutralizing with acids when it is unnecessary to accelerate grain growth.
Therefore, after ammonia is used in the formation of silver halide crystals, neutralization with an acid can prevent crystals thus formed from changing due to unnecessary physical ripening until subsequent chemical ripening with chemical sensitizers and also can eliminate any adverse influence of ammonia on chemical ripening. Further, the neutralized ammonia does not interfere with the adsorption of various compounds added before coating (e.g., sensitizing dyes, antifoggants, stabilizers, etc.) onto silver halide crystals.
Moreover, because ammonia can lose its effect to cause grain growth in the course of the formation of silver halide crystals, it is possible to clearly distinguish between the central portion (core) and the periphery (outer shell) of the silver halide grains in terms of halogen composition. For example, in the case when it is intended to prepare a silver iodobromide photographic emulsion having high sensitivity and good graininess, i.e., when it is intended to form crystal grains comprising a silver iodobromide core having a high iodine content for the purpose of raising light absorption to increase sensitivity or improving graininess and a silver halide outer shell having a low iodine content for the purpose of accelerating development (double layered structure grains), it is required to make the silver halide solvent display its effect only in the formation of the core and to inactivate the silver halide solvent in the formation of the outer shell. This is because if a grain growth effect more than is necessary is encountered in the formation of the outer shell, the iodine in the core penetrates into the outer shell to increase the iodine content of the outer shell, thus resulting in adverse effects such as retardation of development and a reduction in contrast.
However, the method of using ammonia as a silver halide solvent has a strict limitation on the pH levels at which the method is applicable and the problem that the method is apt to increase fog.
As described above, it is only the nitrogen-containing silver halide solvents, e.g., ammonia, that can be inactivated when necessary. It has hitherto been impossible to inactivate the grain growing effect of sulfur-containing silver halide solvents, such as thioether compounds, thione compounds, thiocyanates, etc., by any means but removal by washing with water. However, washing with water to cease the effect of grain growth requires a great deal of cost and time in the production of silver halide emulsions and, therefore, is not suitable as a practical method. In addition, these sulfur-containing silver halide solvents cannot be completely removed by washing with water and remain in emulsions because of their higher affinity to silver halide grains as compared with ammonia. The sulfur-containing silver halide solvents remaining unremoved result in various unfavorable influences, such as on chemical ripening (e.g., fog is increased; chemical ripening proceeds drastically; chemical ripening cannot be easily stopped even by cooling or addition of hydroxytetraazaindene or sensitizing dyes; etc.), an accelerated deterioration in photographic properties during preservation, hindering adsorption of sensitizing dyes in the case of using silver halide solvents of strong adsorption, and the like.
Nevertheless, the sulfur-containing silver halide solvents have many advantages such that a dispersion of silver halide grains having a narrow grain size distribution can be produced more easily as compared with the use of ammonia; iodine distribution in a highly sensitive silver iodobromide emulsion can easily be made uniform; growth of grains can be accelerated at low pH levels; silver halide grains hardly sensitive to pressure can be formed; and the like.
In the light of the above-described circumstances, it has been strongly demanded to develop a system by which the grain growth effect of the sulfur-containing silver halide solvents can be reduced or inactivated whenever necessary like a system employing acids to inactivate ammonia.